The present invention is directed to a method of converting a line voltage to an RMS load voltage in a lamp.
Some loads, such as lamps, operate at a voltage lower than a line (or mains) voltage of, for example, 120V or 220V, and for such loads a voltage converter (or power controller) that converts line voltage to a lower operating voltage must be provided.
The power supplied to the load may be controlled with a phase-control clipping circuit, such as shown FIG. 1, that includes a capacitor 22, a diac 24, a triac 26 that is triggered by the diac 24, and resistor 28. The resistor 28 may be a potentiometer that sets a resistance in the circuit to control the phase at which the triac 26 fires. In operation, a clipping circuit such as shown in FIG. 1 has two states. In the first state the diac 24 and triac 26 operate in the cutoff region where virtually no current flows. Since the diac and triac function as open circuits in this state, the result is an RC series network such as illustrated in FIG. 2. Due to the nature of such an RC series network, the voltage across the capacitor 22 leads the line voltage by a phase angle that is determined by the resistance and capacitance in the RC series network. The voltage across the diac 24 is analogous to the voltage drop across the capacitor 22 and thus the diac will fire once breakover voltage VBO is achieved across the capacitor. The triac 26 fires when the diac 24 fires. Once the diac has triggered the triac, the triac will continue to operate in saturation until the diac voltage approaches zero. That is, the triac will continue to conduct until the line voltage nears zero crossing. The virtual short circuit provided by the triac becomes the second state of the clipping circuit as illustrated in FIG. 3. Triggering of the triac 26 in the clipping circuit is forward phase-controlled by the RC series network and the leading portion of the line voltage waveform is clipped until triggering occurs as illustrated in FIG. 4. The RMS load voltage is determined by the resistance and capacitance values in the clipping circuit since the phase at which the clipping occurs is determined by the RC series network and since the RMS voltage depends on how much energy is removed by the clipping.
With reference to FIG. 5, clipping is characterized by a conduction angle α and a delay angle θ. The conduction angle is the phase between the point on the load voltage/current waveforms where the triac begins conducting and the point on the load voltage/current waveform where the triac stops conducting. Conversely, the delay angle is the phase delay between the leading line voltage zero crossing and the point where the triac begins conducting. FIG. 5 shows the conduction angle convention for forward phase clipping, FIG. 6 shows the conduction angle convention for reverse phase clipping (the conduction angle α immediately follows a polarity change of the load voltage), and FIG. 7 shows the conduction angle convention for forward/reverse hybrid phase clipping (the conduction angles α1 and α2 immediately follow and immediately precede a polarity change.)
Instead of phase-clipping, a suitable RMS load voltage may be established with a voltage conversion circuit that uses pulse width modulation to reduce the energy supplied to the load. Pulse width modulation (PWM) may be achieved with a microcontroller that generates signals (e.g., pulses) whose frequency and duration establish a duty cycle for a transistor switch that is appropriate for the desired RMS load voltage. The signals are applied to the gate of the transistor switch so that the voltage applied to the light emitting element is switched ON and OFF at much greater speed than the line voltage frequency (typically 50-60 Hz). The frequency of the signals is desirably higher than the audible range (i.e., above about 20 kHz). FIG. 8 shows an example of an incoming voltage waveform and a pulse width modulated voltage waveform (the frequency of the PWM being reduced to illustrate the modulation). Phase clipping and PWM are also explained in the U.S. applications mentioned below and incorporated by reference.
Line voltage may vary from location to location or at a particular location up to about 10-15% and may vary more than this in unusual situations. Such variations can cause a harmful variation in RMS load voltage in the load (e.g., a lamp). For example, if line voltage were above the standard for which the voltage conversion circuit was designed, the triac 26 (FIG. 1) may trigger early thereby increasing RMS load voltage. In a halogen incandescent lamp, it is desirable to have an RMS load voltage that is nearly constant.
Further, if the line voltage decreases significantly, the voltage conversion circuit will change the phase conduction angle or switch duty cycle to attempt to maintain the desired RMS load voltage and such changes will increase the current drawn by the load. Increasing load current can overload a system and cause system failure.
For example, a building equipped with a 100 ampere/120V lighting circuit may be loaded up to about 80% of the maximum so that it would be expected that an 80 ampere load would be placed on the circuit. The circuit can power 50 W/120V lamps that each includes voltage reduction circuitry to provide 50V to the lamp filament. At rated voltage, a 50 W/120V lamp draws 0.417 amperes so this circuit could handle about 190 such lamps (80 amps/0.417 amps per lamp=about 190 lamps). If the input voltage drops from the normal 120V to 90V (a 25% drop), the conduction angle or duty cycle would increase to sustain 50 W/50V at the filament. However, in order to supply 50 W with only 90V, each lamp must draw 0.556 amperes, increasing the total draw on the circuit to 106 amperes, probably causing a circuit breaker to trip. Thus, the performance of conventional voltage reduction circuitry in abnormal situations requires improvement.
When the power controller is used in a voltage converter of a lamp, the voltage converter may be provided in a fixture to which the lamp is connected or within the lamp itself. U.S. Pat. No. 3,869,631 is an example of the latter, in which a diode is provided in the lamp base for clipping the line voltage to reduce RMS load voltage at the light emitting element. U.S. Pat. No. 6,445,133 is another example of the latter, in which transformer circuits are provided in the lamp base for reducing the load voltage at the light emitting element.